High temperature cobalt and nickel-based superalloys are widely used to form certain components of gas turbine engines, including combustors and turbine vanes and blades. While high-temperature superalloy components are often formed by casting, circumstances exist where superalloy components are preferably or are required to be fabricated by welding. For example, components having complex configurations, such as turbine midframes and shroud support rings, can be more readily fabricated by welding separate castings together. Because the cost of components formed from high-temperature cobalt and nickel-based superalloys is relatively high, restoring and repairing these components is typically more desirable than replacing them when they become worn or damaged. As a result, welding is also widely used as a method for restoring blade tips, and for repairing cracks and other surface discontinuities in superalloy components caused by thermal cycling or foreign object impact. To improve yields, superalloy components are often welded while maintained at an elevated temperature, e.g., in excess of about 1500.degree. F. (about 815.degree. C.).
Superalloy components of gas turbine engines must generally be thermally stress-relieved before welding to relax residual stresses present from engine service, and then stress-relieved after welding to relax residual stresses induced during cool down from the welding operation. Heat treatment also provides stress relief by dissolution of a portion of hardening gamma prime (.gamma.') in .gamma.'-strengthened nickel-base superalloys. Generally, the heat treatment parameters will vary depending on the alloy of interest, the amount of residual stress relief and dissolution required, furnace design, component geometry and many other factors. The ramping rates, soak temperatures, hold times and cooling rates for stress relief and dissolution heat treatments are critical in order to obtain the desired stress relief without adversely affecting the superalloy and its properties.
In the past, pre-weld and post-weld heat treatments have been performed in large batch heat treatment furnaces to ramp and hold a group of components at a suitable heat treatment temperature. Drawbacks to the use of batch heat treatment processes include long heat treatment times due in part to the mass of the large batch furnace and the mass of the typically large number of components being heat treated. Additionally, long queuing times occur while batches are assembled as individual components are repaired. Therefore, use of batch furnace pre-weld and post-weld stress relief heat treatments represent a time delay to the flow of components through a welding line, and is an inefficient method to metallurgically condition components for welding.
While overcoming the prior art requirement for batch heat treatments, use of the apparatus taught by Broderick et al. has encountered difficulties associated with oxidation of the component being welded. While sealed along its four side walls and base, the enclosure taught by Broderick et al. is required to be open at its top in order to gain access to and weld the component within. Though Broderick et al. flow an inert gas up through the enclosure, and employ an overhead exhaust hood for drawing away the inert gas and flumes generated during the welding operation, oxidation of superalloy components has nonetheless occurred. Accordingly, further improvements to the apparatus taught by Broderick et al. are necessary to improve weld quality and yields, in addition to achieving improved processing efficiency for manufacturing, restoring and repairing superalloy components by welding.
As a solution to the above, U.S. Pat. No. 6,124,568 to Broderick et al. teach an apparatus and method for heat treating and welding superalloy components. More particularly, the apparatus enables pre-weld and post-weld heat treatments to be performed on a component within the same enclosure in which the welding operation is performed. The enclosure is used in conjunction with a memory storage device that stores appropriate pre-weld and post-weld heat treatment temperature profiles and a welding temperature profile for the component. The apparatus further includes a control by which the output of the device used to heat the component is adjusted based on the component temperature and according to the preestablished pre-weld and post-weld heat treatment temperature and the welding temperature profiles.